A stepper motor is a motor wherein its driveshaft rotates in small angular steps rather than continuously. For example, each angular step in a commercial stepper system may comprise rotation of 1.8°. The driveshaft can thus be moved to any position reliably by sending an appropriate number of step pulses. Although stepper motors are generally slower as compared to servo motors, stepper motor systems have several significant advantages and thus are very popular in many industries. In particular, no position feedback information is normally required for either position control or speed control. Furthermore, in normal working states, positional error is non-accumulative. Besides, stepper motors are generally compatible with modern digital equipment.
For these reasons, various types and classes of stepping motor have been used in computer peripherals, automated machinery and similar systems. The cost of stepper systems is significantly lower than that of servo systems, mainly due to the removal of costly position feedback devices and complicated feedback control. Moreover, it does not require the tuning of feedback control which needs extra expertise and effort. Also, the simple hardware and control configuration improves system reliability.
However, one of the most unfavorable features of stepper motors is mechanical resonance, especially at low speeds. The problem is less significant at high speeds as the mechanical system tends to filter out high frequency resonance. Resonance prevents the stepper motor from running steadily at certain speeds and reduces the usable torque. Also, it prevents stepper motors from being used on applications that require smooth low speed motion. Resonance exists even with very fine microstepping control due to motor characteristics.
Various damping techniques have been applied in the art to overcome this problem, including mechanical damping, linearized feedback control and electronic damping.
U.S. Pat. No. 5,659,234 entitled “Vibration reduction for stepping motor by vector angle and vector length modulation” describes a method which reduces the vibration caused by system characteristics (including motor characteristic, speed of operation and stepping noise). On a test rig, the dynamic behaviour of the stepping motor in micro stepping mode is measured by a system comprising a rotation encoder. The measured data are filtered and used to compensate the unwanted speed fluctuations. A correction vector table is found iteratively by observing a velocity ripple. The method is applied to a particular configuration of a system characteristic at fixed speed. It is not applicable to a particular motor model as such and cannot be applied to various configurations and speeds.
U.S. Pat. No. 5,847,535 entitled “Active electronic damping for step motor” discloses a different type of damping method based on sensorless technology. A motor torque output is estimated and is compared with an expected value in real time during operation. Position commands are adjusted accordingly to compensate for the torque ripple.
U.S. Pat. No. 6,040,676 entitled “Passive electronic damping for step motor” describes yet another different type of damping method based on capacitor-inductor electrical interaction. A capacitor with matched value is used to create a virtual short circuit path for the electric frequency in vibration. Vibration energy is thus quickly dissipated.
Nevertheless, prior art damping methods have their limitations. For example, mechanical dampers will wear out with prolonged use. Feedback control requires the use of high resolution position feedback devices. Finally, electronic damping usually requires high order equations and complicated motor characteristics identification. Therefore, the need for an effective and practical damping technique still exists. It would be advantageous to provide a novel damping approach that can be applied to different stepper motor models with variable configurations and operation speeds.